Nanoscale Measurements Nanoscale Measurements in Organic Memory Devices from C 60 in Insulating Polymers
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Nanoscale Measurements in Organic Memory Devices from C60 in Insulating Polymers Alokik Kanwal and Manish Chhowalla Materials Science and Engineering, Rutgers University Piscataway, NJ 08854 ABSTRACT Following our pervious works demonstrating all-organic memory devices based on a nanocomposite material consisting of C60 molecules dispersed in an insulating polymer. In this paper, we will report on conducting atomic force microscopy (c-AFM) measurements from nanosized regions on memory devices. The c-AFM nanoscale measurements show a hysteresis of high and low conductance states, in agreement with our previous reports on macroscopic memory devices. The c-AFM measurements were verified by 30nm gap cell devices fabricated via e-beam lithography, which also showed similar current values. Analysis of our data reveals that the conduction mechanism switches from direct tunneling to Fowler-Nordheim tunneling above a threshold voltage. INTRODUCTION The relative ease of production and low cost of organic electronics [1] has generated tremendous interest in developing an organic memory device. Significant progress has been made in their development [2-4]. These devices successfully demonstrate memory operations at the macroscale; however, there have been few attempts to demonstrate their operation at the nanoscale. Successful operation at the nanoscale is important to achieve the high memory densities that are required to compete with the current technology. Here we expand on our previously reported memory devices based on a nanocomposite material consisting of C60 molecules dispersed in an insulating polymer and demonstrate the memory effect at the nanoscale. The devices consisting of C60 molecules dispersed in polyvinyl phenol (PVP) are investigated using conducting atomic force microscopy (c-AFM). The interface between the AFM tip and the film defines the nanosized contacts. The results are similar to the macroscale devices. In order to confirm the c-AFM results we fabricated 30 nm gap cells with electron beam (e-beam) lithography. The resulting hysteresis is similar to both the c-AFM measurements and the macroscale devices. A lack of scaling is observed between the nanoscale and macroscale measurements, and is attributed to field enhancement due to the AFM tip. EXPERIMENTAL DETAILS PVP polymer is electrically semi-insulating and easily dissolves in isopropyl alcohol at ratio of 25mg to 1ml. The C60 fullerene molecule is a closed cage molecule with a precise diameter (0.72 nm) consisting of 60 carbon atoms and is co-dissolved with PVP. Both PVP and 2 C60 were obtained from Sigma Aldrich. Our macroscopic memory devices (~1 mm ) are fabricated using a MIM (metal-insulator-metal) structure, utilizing a cross-point array architecture by sandwiching the storage medium (PVP + 5 wt % C60 blend) between two Al electrodes. The spin coated organic films (30 nm thickness) are well adhered and stable for
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several months. For c-AFM measurements, the organic films are spin coated on ITO coated glass subst
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